One way of transferring a pattern on to a surface by e-beam lithography is to use a variable shaped beam or VSB to reveal a positive or negative resist coating. For doing so, during a fracturing step, the pattern is to be cut into elementary forms to which a radiation dose is assigned (known as “shots”). Geometry and radiation dose of the shots are closely interrelated because, at the dimensions which are now used (technologies with a critical dimension or “CD” of less than 25 nm), the proximity effects (forward scattering and backward scattering) largely depend on the density of the exposed area.
The patterns to be transferred are quite often of a simple geometric form, such as thin rectangles (lines) or squares (interconnections). In these circumstances, the geometry of the shots is defined accordingly and is also simple: each pattern is fractured into a union of rectangular or square shots.
Nevertheless, for a number of applications (inverse lithography, photonics, metrology calibration, etc . . . ), it may be necessary or advantageous to include in the design patterns which are not simple forms of the type previously described, but which may be circles or of an indeterminate, possibly curvilinear, form (further referred to as free-form)
It will also be advantageous to be able to transfer on a mask resolution free-form assist features to better correct the proximity effects in the case of a very low CD.
Under these circumstances traditional fracturing is not advantageous because it generates a very high number of shots, especially when pattern fidelity is critical. The writing time increases proportionally to the number of shots, which in turns increases significantly the cost of producing masks or wafers.
Moreover, independently of the lithography tool, traditional fracturing generates a huge amount of data which is challenging for data storage and transfer. This amount of data is also a critical issue when computing proximity effect correction.
It would therefore be advantageous to use a fracturing method capable of adapting itself to indeterminate forms of patterns to be transferred onto a surface, while taking the maximum benefit of the capacities of the e-beam equipment and software. An attempt in this direction is discussed in U.S. Pat. No. 7,901,850 which discloses an assembly method wherein rectangular and/or triangular shots are assembled in glyphs and superposed in order to pave circular or curvilinear patterns.
But this prior art document fails to disclose a solution which would be providing a fracturing methodology capable of addressing various operational constraints such as contour roughness and/or resolution targets and/or the geometries of shots available on the e-beam equipment.